This invention relates to a heat-resistant, optically-black coating that is adherent to metal surfaces. The coating is particularly useful for reducing the reflection of visible and ultraviolet light from the inner surfaces of an exposure lighthouse that is used for producing mosaic-type viewing screens for cathode-ray tubes.
Exposure lighthouses that are used for producing mosaic-type viewing screens for cathode-ray tubes are described in the prior art; for example, in U.S. Pat. Nos. 2,942,099 to N. R. Goldstein, 3,592,112 to H. R. Frey and 3,970,456 to T. W. Branton. Such lighthouses generally include a metal light box, a cooled, small-area lamp which emits both visible and ultraviolet light and optical components for reflecting and refracting light from the lamp to a photosensitive layer on a support for the viewing screen. Except for the optical components, it is desirable that inner surfaces of the lighthouse carry a light-absorbent coating that reflects a minimum of light that is incident thereon in order to minimize light scattering. In this application, the coated inner surfaces must absorb both visible and ultraviolet light, preferably in the range of 2000 to 7500 .ANG.. Because some lighthouse parts reach temperatures above 700.degree. C., the lighthouse parts are made of refractory metal, usually steel. This requires that the light-absorbent coating be chemically stable up to at least 750.degree. C. and adherent to metal. Coatings with organic binders are unsatisfactory because they are degraded by temperatures above about 400.degree. C. Because the inner surfaces involve a plurality of parts, it is desirable that the coating can be applied either before or after the parts are assembled.
With the evolution of high-resolution display cathode-ray tubes, it has become more important to minimize light scattering in the lighthouses. Light scattering is a cause of poor uniformity and cross contamination in the viewing screens produced on the lighthouse. Previous black coatings on steel or other metal surfaces are unsatisfactory because they lack at least one of the above-described characteristics.